The present invention relates generally to global positioning systems (GPS), and more specifically to improved signal detection acquisition time and low level signal detection for GPS receivers.
One drawback of GPS is often weak signal detection, acquisition time and energy use for operating power. Weak signal detection occurs in places where there is attenuation caused by buildings, trees, or other objects and, further, because the GPS signal is generally weak and received from a satellite at an extraordinary distance. Trees, buildings, and other high-profile objects may cause line of sight interference resulting in the problem of weak or low signal detection.
Conventional GPS signal processing techniques often have problems with bandwidth and signal power. The GPS satellites transmit a relatively weak signal, guaranteed signal levels are only −130 dB-m on the earth's surface, however actual signals as measured on the earth's surface are conventionally about −125 dB-m. The acquisition threshold of some conventional consumer grade GPS receivers are on the order of −137 dB-m, thus the link margin for signal acquisition is only about 7 to 12 dB.
Conventional GPS receivers generally use a sequential detection algorithm to acquire the code division multiple access (CDMA) signals. The acquisition threshold to lower levels may be achieved by lengthening the pre-detection integration (PDI) interval at the expense of acquisition time. Even so, there is a maximum PDI of about 10 milliseconds beyond which the sequential detection process breaks down. This is because the GPS signal structure includes BPSK modulated navigation data (50 BPS) transmitted on top of the 1.023 MHz spreading code that ultimately limits how long one can coherently integrate in order to increase the signal-to-noise ratio (SNR). Beyond 10-20 milliseconds the data bit transitions cause the integration sum to be reduced or go to zero, depending on the phase relationship of the integration period relative to the data bit transition.
Conventional GPS receivers often may be embedded within portable devices, where energy usage is of paramount importance. Such devices include, but are not limited to, mobile telephones, handheld computers, portable computers, surveying devices, and other devices that make use of information provided a GPS receiver. When these embedded GPS receivers operate, they consume a substantial amount of energy, which depletes energy from the battery that could be made use of by the co-embedded functions. If GPS correlation can be done faster, battery energy can be conserved because the GPS receiver may be turned off when correlation is achieved.
Accordingly, there is need for an improved GPS signal acquisition method and system that may operate with weaker signals and may be configured to lock on to satellite signals faster than conventional systems.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.